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Cause analysis of marine propeller failure

Views: 6     Author: Site Editor     Publish Time: 2020-04-18      Origin: Site

A ship's propeller transmits the thrust it produces to the ship itself, which is the part that converts the Torque produced by the engine into kinetic energy. If the propeller breaks down, the normal movement of the ship will be affected to different degrees. When different faults occur, the treatment methods are different, and the proper method should be based on the reasonable analysis of the faults, that is to say, the cause analysis is also very important. The accuracy of the cause analysis directly affects the result of judging the type of fault. The Fault of the Marine Propeller winding the fishing net, the propeller is often wound by the fishing net, this situation will increase the Torque of the propeller after forensic examination, thus causing the increase of the engine load, when the overload is carried out, and may stop the shaft system or propeller, so that the shaft system and propeller blades damage, deformation, and sometimes vibration noise will increase shafting. When the shafting appears the shock signal based on the rotational speed pole, it is proved that the mechanical operation of the shafting is not normal. In this case, the shafting or the propeller need to be further checked to find out the cause of the shock signal, it is possible to have the following failures: 1. The fin of the hood is deformed. 2. Wear or damage of Stern Shaft Flange. Damaged or missing stern bearing fixed hexagon bolt. Coupling flange displacement occurs 5. Thrust bearing reversing block damaged 6. The vibration and noise of the shaft system increase, which is caused by the fishing net debris blocking the cooling water passage of the bearing, and the shaft coupling Flange displacement which makes the shaft system parts loose. Due to the complicated working environment of the stern shaft, the vibration of the shaft system is caused by the friction between the propeller and the accessory parts, and the poor lubrication effect and the imperfect clearance between the bearing and the stern shaft, it is also possible that the sound is caused by the incorrect installation of the sealing device, the contact between the propeller blade and the shroud, the stern shaft shroud and the propeller hub. The contact point of the propeller and its appendages may be at the propeller hub and the Stern Shield, and in the docking room, the shield near the hub shall be properly removed to ensure sufficient separation, so there's no more contact friction. The unbalance caused by the propeller blade breaking, when the ship hull is running on the calm river surface, when the ship hull vibration occurs in an instant, the crew will generally slow down the voyage, the vibration will be weakened, continue to slow down, vibration becomes smaller. Analysis of the causes of this situation: This situation is likely to happen is the propeller blade has been broken, so that the propeller out of balance, so that the hull of the violent vibration. Generally this situation may be due to poor propeller material performance, casting defects, or the blades of the propeller are seriously corroded, polluted by living things in the water, or the blades touch the cables, rocks and other objects, causing the blades to be denuded, broken, deformed, etc. , which causes the center of gravity of the blades to be unstable and out of balance, striking the stern shaft causes severe vibration of the shafting or hull. In order to solve this problem, we should repair the broken blade by means of patch, repair the surface crack, cavitation erosion and corrosion systematically, and correct the deformation. To static balance, the pitch of the propeller, such as quality testing, and to meet the relevant norms. When the propeller is in operation, the force between the Blade and the water causes the water to reaction against the hull, which in turn causes the hull to advance. In the long-term use of the blade, due to the material is not used, fatigue limit is not used, material deformation and fracture are over time until such a situation, and in addition to this kind of force, there are still many uncertain factors, such as water erosion makes the blade thickness thinning, this thickness thinning makes the blade deformation increased. The sound of propeller is caused by the frequency of the random vortex emitted by the blade and the frequency of the structural response of the maximum amplitude of the blade vibration. Here are some ways to tone it down: 1. Trim the trailing edge of the blades. To change the emission frequency, first of all, the thickness of the trailing edge of the blade should be changed. The structure of the trailing edge affects the frequency and intensity of the emission vortex, and determines the ratio of the transverse and Longitudinal spacing of the Vortex Street. However, the sole purpose is to eliminate the singing sound, because the tail edge is weakened too thin will affect its strength, resulting in a reduction in strength to meet the needs of the work. 2. Increase Blade damping. Using highly damped materials to make propellers also eliminates the hum. 3. Near the back of the blade, about 1 / 15 of the chord length of the section, a few small metal protrusions are positioned equidistant along the radial direction and made into a semicircle. 4. The blade with the edge into a sawtooth-shaped structure, tooth-shaped height for the chord length of about 7% , tooth-shaped bottom edge and tooth height is equal. Propeller Cavitation, propeller in the work, if the cavitation near its surface, blade material will be subjected to hydraulic impact of long-term sustained force, resulting in pitting corrosion. Cavitation often produces vibration and noise, and there will be spot corrosion on the metal surface. In severe cases, the Dead Blade will be perforated or broken, resulting in instability of the whole propeller center of gravity, vibration and noise. The solution to these problems is as follows: 1. Improving the design of blade structure, increasing the flow area, reducing the thickness of blade, so that the shape of blade becomes streamline shape. 2. Increase the inlet angle of the blade, reduce the curvature of the blade, thereby reducing blade obstruction. 3. Blades are replaced with cavitation resistant materials, such as materials with high hardness, strength, toughness, and good chemical stability. The Fault of marine propeller is a kind of normal fault. Because of the difference in the working conditions of different ship propellers with different materials, it is not the same as Germany to analyze the cause of the fault, the actual water surface condition and the working condition of the parts around the propeller are taken into account when dealing with the fault and analyzing it, so as to avoid judging the fault cause by experience.

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